We can see the “physically possible” element is relevant in the way “setting” works in an RPG. For example, even in very freeform fantasy games nobody is going to have their character conjure up a communicator to ask the Starship Enterprise to beam them up. It just doesn't even occur to you as a possibility for something you would have your character do, so it never enters play.
We have clearly played with different groups because I've played with more than a few who would be perfectly happy to do that. Even more than that who would be perfectly happy to negotiate that being one of the basic premises of the game.
Historically, in terms of RPG development, earlier games were much, much less tied to "what is canon" in terms of possibility compared to the modern take on the traditionally GM-led architecture. Elements of science fiction have cropped up all through D&D in its various incarnations. Literature literally based on the author's D&D campaigns (Steven Brust comes to mind immediately with the Vlad Taltos series) contains a fairly good chunk of aliens and hyper-tech as central underpinnings of the world.
So we have to revisit what it means when you say "we can see that the physically possible element is relevant in the way setting works in RPG." We have to contextualize that as saying, "when we are describing the sorts of things that can happen in a particular setting," and differentiate that from "the set of things which can happen in a particular setting."
Otherwise you've just invalidated the existence of fairly central elements of a large chunk of RPG experience, and that's inherently wrong.
But it didn't even occur to the protagonist's player that not going on this “side-quest” was an option. To him, the idea that his character now needed to climb the mountain may as well have been a law of physics.
This isn't a problem with the player's understanding of the world. This is a side effect of a poorly trained player, one who has been conditioned to listen to whatever the GM says and accept that the job of the GM is to put stumbling blocks in the way of the characters – and that, inherently, if they don't accept whatever the GM says, whether it be the description of the world or the presence of a "quest", that's simply the way it is. What that player did was perfectly sensible, having been trained on extremely traditional, GM-led role-playing.
This is an issue of expectation among the players being unaligned with that of the GM. Having no narrative power for the majority of goodly chunks of the hobby, why should a player expect that they could deny the GM his side quest? There is no Chekhov's gun in that mode of play – or rather, everything is Chekhov's gun and thus nothing is.
From the player perspective, maybe he did think about skipping out on this side quest, but then he would have to listen to the GM bitch about having invalidated two months of writing and scenario design in five minutes, and that's just a lot put up with.
In the real world we're mostly constrained by our sense of morality, but in an RPG we're often playing a specific character, so it might be the case that our sense of “who that character is” in terms of personality, etc., constrains us in terms of what we consider possible.
Facts not in evidence, Your Honor.
That is a big assertion to make, and not one that I can really think is based on any sort of empirical experience. In the real world, according to people, they're mostly constrained by the sense of what they can get away with, how they will be seen by an external factor, and somewhere down the list the things that they want – and on that last list is, for some people, "be a moral person." I make a natural 20 on my roll to disbelieve that in the real world we are mostly constrained by our sense of morality. I would be much more inclined to be supportive if you said that "in the real world, we are mostly constrained by our sense of aesthetics."
Also, I think you're not using the term "constraint" with restraint. Especially in light of the fact at the beginning of the post, you pointed out that people often use the can/cannot dichotomy to refer to things which definitely can happen, and which may be better phrased as should/should not.
"Particles shouldn't go faster than the speed of light, so if you see it happening or detect it happening, something is either wrong with your sensor, your calculations, or you've discovered something amazing."
"One shouldn't complete an entire career in research without making a few mistakes. If you see that someone has, someone is probably lying to you."
"You shouldn't keep treating your sister that way – look at how upset she is! And if you do, there will be repercussions because she knows where you sleep and likes sharp things."
This analysis in general seems to confuse a lot of the medium for the message. It assumes that "can't" is a hard and fast when in actual practice it is as probabilistic as "maybe" and "shouldn't" and "might not", simply additionally burdened with a layer of intent by the speaker which effectively says "I don't want you to."
Confusing connotation and denotation in the shades between is a particularly academic mode of failure, but I think we're seeing it on display here.
(It might be especially relevant when considering how to make games accessible to beginners, sometimes old hands at RPGs can have a hard time imagining what a game looks like from the perspective of someone who doesn't know what they're “allowed” to do.)
It's funny, because I find new players who don't yet know what they are "allowed to do," and thus feel free to try anything, to be the best kinds of players at my table. I've spent the last five years or more in my gaming teaching people to break those aesthetic habits that they've been in, to play games in which narrative power is in everyone's hands to some degree, where a central authority is not something you have to look to to resolve conflicts, where stories are seen as an emergent property of a series of experiences rather than something imposed on a group of other people, and in general treating people like interesting, creative agents in the context of the game.
There are games that do that. At this point, there are a lot of games that either move toward that space or have set up a colony and are rapidly reproducing in it.
If anything, this article reinforces something that story games have been really aggressively clear about pursuing for the last several years – taking the time, architecturally, mechanically, and in the text, to get everybody at the table on the same page about their expectations of the setting, and bringing them to an awareness that yes, we can step outside these bounds it it's okay. Yes, we can introduce elements which differ from others, and that's okay. Yes, you as a player have the power to shape your experience, and that's okay.
This "Session 0," as the kids seem to be calling it these days, is a big deal and it helps a lot no matter what kind of game you're running. If we take anything away, it should be that.
I'm interested in the distinction you draw between morals and aesthetics. You think they're different things?
ETA: What I mean is, morals seems like a subset of aesthetics.
In denotation, I think they are exactly the same in practice. In connotation, they are vastly different things.
Morals are given a privileged air of veracity. Once something is declared to be someone's moral (or ethic), socially it moves from something that we can discuss and debate at the same level of "which do you prefer, fried or baked chicken?" to "what do you mean, you think I over-value human life?"
Which is, of course, quite silly.
Morals are just really loudly stated aesthetics, and anyone who tells you differently is making an appeal to a third person authority whom you didn't take on board. Some of them are useful (as generally finding murder inaesthetic and promoting that as a social norm can be useful in keeping yourself from being murdered), but that is by no means the nature of things inherently.
But just because they are at heart the same thing to reasonable people, that's no reason to lose track of the fact that most of the time you are dealing with unreasonable people – and these unreasonable people believe that morals are qualitatively different from aesthetics, that they can't be challenged, that they shouldn't be challenged, nor should they be argued either for or against – because argument implies that rational thinking can be applied to the question, and in their minds there can be no question.
You'll notice that we have somehow ended up back at the "can/can't" versus "should/shouldn't," and that is not a surprise. "Can/can't" is often complicated when it runs in the reality which only cares about "does/doesn't."
So, no – I don't think that morals and aesthetics are different things, but I would be a fool not to recognize that other people react to them as though they were different things, they behave as though they were different things, and even though I disagree with those decisions and those choices, if I want to communicate with them I have to acknowledge the difference in approach.
Otherwise, how would I mock them properly?